1. Field of the Invention
This invention relates to a method for applying various coating liquids to the surface of a long travelling support (hereinafter, for simplicity designated a "web"). More particularly, the invention relates to a method for applying various coating liquids to the web while preventing a thick coating of the coating liquid which often is formed in a connection between webs.
2. Description of the Prior Art
A conventional process, wherein various coating liquids are applied to a web such as plastic films, paper, metallic sheets, etc., and then dried to obtain desired products, has been widely used in manufacturing photographic films, printing papers, magnetic tapes, adhesive tapes, no-carbon papers, PS formats, etc.
In the process of manufacture as described above, it is advantageous for the coating to be applied in a substantially continuous manner, and in general the coating operation is usually carried out in such a way that a web formed by connecting pieces of webs or web portions having a limited length one after another is fed to a coating station so that the coating operation occurs without interruption. In connecting web portions, the so-called "butt joint" is preferably employed, in which ends of both of the web portions are abutted and connected using, e.g., an adhesive tape as a splicing tape; and it is known to be more advantageous to apply the splicing tape to the surface on which the coating liquid is to be applied than on the opposite surface of the web, and it is known to pre-apply a surface treatment to the splicing tape so that the surface characteristic of the splicing tape will be substantially the same as that of the surface of web to which the coating liquid is applied.
However, if a fluid coating liquid is applied to the web splice thus formed in a continuous manner, several significant disadvantages, which have been heretofore encountered, are produced principally following, i.e., downstream of, the splice.
One disadvantage is such that bubbles are formed due to the presence of air between the web surface directly following the splice and the coated layer, and these bubbles adhere to coating nozzles so that the coated surface over a considerable length is adversely affected by streaks or the like. Another disadvantage is that uncoated or extremely thinly coated portions and thereafter locally thickly coated portions are formed in the coating applied to the surface of the web directly following the splicing tape due to the step-like discontinuous changes at the trailing edge of the splicing tape in the splice and the presence of air as described above. A considerable amount of time is required to dry the thickly coated portions in a subsequent drying process as compared with the other portions properly coated. If the drying process does not have sufficient capacity to dry the thickly coated portion, the result is that the thickly coated portion remains undried, the undried coated liquid is transferred onto rollers or the like to contaminate the equipment and thereby damage the coating properly applied, leading to fatal defects in the product produced. For this reason, where the equipment is contaminated, the manufacturing operation must be stopped for cleaning, which results in a marked reduction in production efficiency. Conversely, where the undried portion is to be prevented from occurring, sufficient drying capacity for the locally thickly coated portion must be provided in the drying process. This thickly coated portion, however, is not useful as a product but is discarded, and therefore it is extremely uneconomical to spend money for drying those portions ultimately to be discarded.
While the splicing of web portions has been described above, similar coating failures such as coating irregularities also occur if the surface of the web to which a liquid is to be applied has other types of discontinuities such as projections, steps, or the like.
Several proposals have been made to prevent the aforementioned coating failures caused by discontinuities formed by the splicing tape or the like.
One such device is disclosed in U.S. Pat. No. 3,531,362. Briefly, this method comprises (1) applying an oily hydrophobic material to the discontinuities, the surface of the web adjacent thereto, and the following portion of the discontinuities, or (2) providing an inclination to the discontinuities so as to eliminate the wedge-like space formed in the trailing edge of the discontinuities prior to applying the coating liquid to the web.
However, in the above-described method (1), generally, if the coating liquid is aqueous in nature, the oily hydrophobic material tends to produce a poor adhesion relative to the coating and the coating after drying tends to peel off by application of a small external force. Furthermore, this method requires an operation wherein a surface treatment is applied using an oily hydrophobic treating liquid to the discontinuities. However, this operation tends to result in equipment contamination by the treating liquid and a contamination of other portions of the web. Moreover, a dryer must be provided if it is desired to sufficiently dry those portions to which the treating liquid has once been applied.
In addition, the above-described method (2) is not practical in terms of the actual shape and thickness of the splicing adhesive tape. That is, splicing tapes generally used for this purpose are made by applying an adhesive to elements having a thickness of about 10 to 50 microns, with most tapes having a total thickness formed by the element plus the adhesive of about 30 to 100 microns. In order to effectively use this method, the adhesive portion should also be inclined as well. However, it is very difficult to incline the trailing edge of such a thin tape either in pre-treatment or in treatment after the tape has been applied.
Also, the method wherein an inclination is formed by inserting a packing such as rubber cement between the trailing edge and the web surface after the tape has been applied is complicated and at the same time other portions of web and the equipment tend to be contaminated similarly to the afore-mentioned treatment using the oily hydrophobic material.
Another method is disclosed in British Pat. No. 1,243,663. This method comprises pre-spraying and adhering water to at least the trailing edge of a splicing tape, and applying the coating liquid before the water has completely dried.
However, this method possesses several disadvantages in that (a) a complicated device is required to detect the splicing tape immediately before a coating station to apply water to the trailing edge of the splicing tape; (b) after water is applied to the web, the web can not be supported, for example, by rollers, etc., so that the layout of the web passage in the vicinity of the coating station is limited; and (c) where the web absorbs water with difficulty, water drops on the web adhere to coating devices such as coating nozzles, thus adversely affecting the coating operation thereafter.